Subplasmalemmal linear densities, consisting of a thin layer of electro-dense material immediately subjacent to the inner aspect of the plasma membrane, were found in cells of the mononuclear phagocyte system in lung biopsies from patients with fibrotic disorders. Two types of subplasmalemmal linear densities were found: impaired and paired. The latter formed junctions that differed morphologically from other types of junctions. Such junctions may contribute to the aggregation and immobilization of mononuclear phagocytes.